1. Field of the Invention
The present invention relates to processes for producing mercury cadmium telluride and, in particular, to processes applicable to the production of such materials having a unitary crystalline structure and to the product obtained thereby.
2. Description of the Prior Art
During the past decade, mercury cadmium telluride has gained increasing prominence among materials suitable for the fabrication of photodetectors and the like. The detector qualities inherent in this ternary compound may be traced to its "pseudo-binary" character. That is, both mercury and cadmium are known to behave as though they were the only element in combination with the tellurium. Thus, a photodetector material is presented which consists of a mixture of cadmium telluride, a wide gap semiconductor (E.sub.g =1.6 eV), and mercury telluride, a semi-metal having a negative energy gap of about -0.3 eV. The resultant alloy is found to have an energy gap which varies approximately linearly with x, the mole-fraction of cadmium telluride in the alloy. Thus, with proper selection of the fraction x, electronic responses may be obtained over a wide range of infrared wavelengths, a very desirable photodetector characteristic. High performance HgCdTe detectors have been achieved for wavelengths from about 1 to 30 micrometers.
Optimum detector performance is achieved when the detector is formed of monocrystalline material. Such material features a regular geometrical lattice throughout as opposed to polycrystalline material in which grain boundaries of various orientations may act as recombination sites for electrons and holes, thereby reducing the lifetime and detector performance.
A presently favored method of manufacture of monocrystalline Hg.sub.1-x Cd.sub.x Te involves the compounding of a hot liquid mixture (of selected cadmium telluride mole fraction) followed by a quenching process wherein a solid ingot is produced. The ingot is then treated by any of a number of processes loosely referred to as "solid state recrystallization". These may include a variety of methods of applying sustained heat to the ingot for a number of weeks. The quenching and compounding process is discussed, inter alia, in articles by E. Z. Dziuba ["Preparation of Cd.sub.x Hg.sub.1-x Te Crystals by the Vertical-Zone Melting Method", Journal of the Electrochemical Society 116, 104-106 (1969)], L. N. Swink and M. J. Brau ["Rapid, Nondestructive Evaluation of Macroscopic Defects in Crystalline Materials: The Laue Topography of (Hg,Cd)Te", Metallurgical Transactions 1, 629-634 (1970)], T. C. Harman ["Single Crystal Growth of Hg.sub.1-x Cd.sub.x Te", Journal of Electronic Materials 1, 230-242 (1972), J. Steiniger ["Hg-Cd-Te Phase Diagram Determination by High Pressure Reflux", Journal of Electronic Materials 5, 299-320 (1976)], and G. Fiorito et al. ["A Possible Method for the Growth of Homogeneous Mercury Cadmium Telluride Single Crystals", Journal of the Electrochemical Society 125, 315-317 (1978)]. Discussions of solid state recrystallization are found in articles by M. J. Brau et al. ["The Preparation and Electrical Properties of HgCdTe Alloys", Journal of the Electrochemical Society 117, 95C, Abstract No. 87 (1970)], and J. Steininger ["High Pressure Reflux Technique for Growth of Hg.sub.1-x Cd.sub.x Te Crystals", Journal of Crystal Growth 37, 107-115 (1977)]. See also U.S. Pat. Nos. 3,622,399 and 3,468,363.
Economical manufacture of mercury cadmium telluride photodetectors demands that production processes yield maximum single crystal material. Additionally, it is desirable, though not essential, that the crystal produced approach homogeneity (constant x) throughout the length of the grown crystal. Alternatively, detectors of varying radiation sensitivities can be produced from various "slices" of a single crystal of Hg.sub.1-x Cd.sub.x Te.
The quantity of single crystal material eventually grown (via solid state recrystallization) is quite dependent upon the character of the ingot which acts as its "seed". The recrystallization growth process occurs as a result of the decrease in the free energy of relatively high energy polycrystalline material to that of lower energy single crystal material. Maximum single crystal material is obtained when the ingot's microscopic structure consists of a large number of relatively high free energy crystallites.